This invention concerns a sealing element for sealing a pressure piston which is movably arranged in a housing of a brake cylinder, it being possible to insert said sealing element into a ring-shaped groove which is provided in the housing. The sealing element includes an outer sealing lip for contact with a floor of the ring-shaped groove, an inner sealing lip for contact with the pressure piston, and a sealing element back. The invention also concerns a brake cylinder, in particular for hydraulic motor vehicle braking systems, into which such a sealing element for sealing a movably arranged pressure piston is inserted.
A sealing element and a brake cylinder of the above-mentioned type are described in, for instance, WO 2005/019007, and corresponding US Publication No. US 2006/0219507 A1, both of which are incorporated by reference herein. When the pressure piston is moved, and consequently hydraulic pressure is built up in the pressure chamber of the brake cylinder, the back section of the sealing element is pressed onto the side wall of the ring-shaped groove into which the sealing element is inserted. In this way, in addition to the dynamic sealing of the movably arranged pressure piston by the inner sealing lip and the sealing of the ring-shaped groove by the outer sealing lip, an additional, at least temporarily permanent sealing area is created. This area, which fulfils a sealing function between the sealing element back and the groove wall, at least temporarily prevents, in the case of the return movement of the pressure piston when the brake is released, fluid flowing out of the fluid reservoir through the groove into the pressure chamber (so-called overflow), to generate the necessary pressure balance between the pressure chamber and the fluid reservoir. Fluid cannot flow through the overrun holes, which are provided in the pressure piston, into the pressure chamber of the brake cylinder until the overrun holes are no longer sealed by the inner sealing lip of the sealing element.
If the brake is abruptly released by rapidly taking the brake pedal back, the temporary result, because of the rapid return movement of the pressure piston, can be a rapid pressure drop inside the pressure chamber of the brake cylinder, since between the sealing element back and the wall of the groove a sealing area remains, and prevents an overflow of the sealing element for the necessary pressure balance. In this case, the result can be a clear low pressure in the pressure chamber compared with the atmospheric pressure in the unpressurised fluid reservoir. This pressure difference between the pressure chamber and the fluid reservoir is balanced as soon as fluid can flow out of the fluid reservoir through the overrun holes, which are provided in the pressure piston, into the pressure chamber of the brake cylinder. Because this pressure balancing occurs very rapidly, flow noises can occur, and be audible in the passenger compartment. This pressure balancing can also be accompanied by cavitation effects, which can result in increased wear or damage.
From DE 10 2004 057 253 A1, and corresponding U.S. Pat. No. 7,104,059, both of which are incorporated by reference herein, a sealing collar for sealing a pressure piston which is arranged in a main cylinder of a vehicle braking system is known. The sealing collar back has continuous projections or grooves, to prevent the sealing collar sticking to the wall of the groove when the pressure piston is rapidly reset.
Another sealing element in ring-shaped form is described in the published version of the German patent application DE 196 15 157 A1. The ring-shaped sealing element is intended for a central valve of a main cylinder of a hydraulic braking system, and has a first part of an elastomer material and a second part of a rigid material. The elastomer first part includes dome-shaped elevations, which are each provided precisely over pin-shaped continuations of the rigid second part, and run in an axial direction. The purpose of the dome-shaped elevations, in the case of a drive slip regulation cycle, is to open the central valve even against a high overpressure, reliably and conforming to a precisely defined opening instant in relation to the position of the piston.
The invention is therefore based on the object of providing an improved sealing element for a brake cylinder of a motor vehicle braking system, said sealing element avoiding the flow noises which occur when the brake is rapidly released because of the rapid pressure balancing between the pressure chamber and the fluid reservoir, and continuing to ensure sufficient sealing between housing groove and pressure piston if the sealing function of the outer sealing lip fails.